Basic VLAN Question

[fakiee]

Hi,

My understanding of VLANs is that each one is a seperate network from the any others (aauming the ports are all untagged). So for example if I have:

VLAN 100

ip: 10.10.100.1

VLAN 200

ip: 192.168.200.1

These 2 networks should not be able to communicate with each other, right?  So i couldnt ping VLAN 200 while my PC is connected to a port on VLAN 100.

Sorry for the basic question, but my switches at work can all interncoonect with each other VLANs even tho thre is no port tagging between them.

Thanks for your help

[parnassus · Accepted solution]

Hi! VLANs are broadcast domains that means that all packets with a particular VLAN id tagging (IEEE 802.1q) share the same domain; packets with different VLAN id tagging belong to different broadcast domains and can't be exchanged between hosts without the action of a router.

An host (VLAN unaware) connected to an access port which is member of a particular VLAN id (let me say your VLAN id 100) can't connect to another host (VLAN unaware) connected to an access port which is member of a particular different VLAN id (let me say your VLAN id 200)...no matter the IP addressing those hosts have set.

If your switch is capable of acting as an IPv4 Router (this means that IP routing feature is available and enabled) and you assigned two non overlapping IP addresses to your VLAN id interfaces then you're dealing with a routing switch where Routed VLAN Interfaces are operating and thus VLANs are Layer 3 routed each others (at that point your two VLAN unaware hosts on different subnets can finally reach each others if implemented ACLs permit that of if no ACL are used against those VLAN ids).

The switch you have at work is probably a Routing Switch with IPv4 Routing enabled and involved VLAN ids have their respective Layer 3 Routed VLAN Interfaces, at work your hosts use as default gateways the IP addresses assigned to RVI (so hosts use your Routing Switch as their Default Gateways on various VLAN ids).